The present invention relates to a method for etching patterns on, or removing oxide scale from, substrates with hydrofluoric acid (HF) compositions. More particularly, the present invention relates to etching or removing oxide scale from surfaces with a polymer-thickened HF composition.
The glass and electronics industries use large quantities of aqueous HF to etch patterns on different substrates. The metal finishing industries also use large quantities of aqueous HF to remove metal oxide scale during stainless steel production (pickling). Because of the relatively low viscosity of aqueous HF, it is difficult to contain the aqueous HF to the areas where it is applied.
Thus, etching processes employing HF compositions require the use of a mask or resist, particularly when the purpose of the etching is to form a pattern on a surface. With resists and masks, any pattern that is being applied must be fixed, and as a result, varying patterns are precluded.
The low viscosity of aqueous HF solutions also pose a problem for metal oxide removal methods. Maintaining the HF composition in contact with the oxide layer for a sufficient period of time to remove it is problematic, particularly with vertical surfaces.
Thus, a need exists for HF compositions for etching and oxide scale removal HF that do not suffer from the disadvantages of the prior art aqueous HF compositions.
These needs are met by the present invention. The invention provides methods for etching and metal oxide scale removal using an HF composition that can be applied as a liquid, paste or gel exactly in the places needed for the specific time needed, and then removed. The HF composition has a viscosity and surface tension that is effective to retard the flow of the material. This eliminates the need for masks or resists when etching, and also increases the surface contact of the HF in the composition with oxide layers to be removed.
The present invention thus provides a method of etching a substrate by applying in the form of a predetermined pattern to the surface of the substrate an HF composition containing a polymer and an amount of HF effective to etch the surface, until the pattern is etched into the surface, and optionally removing the composition.
The present invention also provides a method for removing oxide scale from an oxidizable surface by applying to the oxide scale an HF composition containing a polymer and an amount of HF effective to remove the oxide scale, until the oxide scale is removed, and optionally removing the composition.
In both embodiments, the HF composition may be either rinsed or wiped away with water. Alternatively, the substrate may be heated to remove the HF from the composition, or the composition may be neutralized to stop the etching or oxide scale removal process, before the HF composition is rinsed or wiped away with water. The composition may be recovered from the water, reconstituted with HF, and reused.
The polymer produces high viscosity etching and oxide scale removal compositions that are resistant to flow. Preferably, the polymer is inert, or substantially inert, to HF. The polymer may be a homopolymer, copolymer, or mixtures thereof.
Generally, the polymers used in the invention have weight average molecular weights from about 5,000 to about 10,000,000 daltons. Preferably, polymers with molecular weights are from about 5,000 to about 1,000,000 daltons. A molecular weight between about 50,000 to 500,000 daltons is most preferred. Particularly preferred polymers are water-soluble. xe2x80x9cWater-solublexe2x80x9d means any polymer that swells to at least twice its dry volume, or dissolves, with the addition of an excess of water at room temperature.
Water-soluble polymers suitable for use with the present invention include semi-synthetic water-soluble polymers, synthetic water-soluble polymers, and mixtures thereof. Semi-synthetic water-soluble polymers are naturally-occurring water-soluble polymer derivatives. Synthetic water-soluble polymers are water-soluble polymers or their derivatives that are not naturally-occurring. (They are formed only through chemical reactions.)
Both the semi-synthetic and synthetic water-soluble polymers are commercially available. Exemplary commercially-available semi-synthetic water-soluble polymers include, without limitation, cellulose ethers, modified starches, starch derivatives, natural gum derivatives, and mixtures thereof. Illustrative commercially-available synthetic water-soluble polymers include, without limitation, polymers, related polymers, and polymer salts of acrylamide, acrylic acid, ethylene oxide, methacrylic acid, ethacrylic acid, esters of acrylic, methacrylic, and ethacrylic acid, polyethyleneimine, polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, and copolymers thereof, as well as copolymers with other monomers, for example, substituted and unsubstituted two to eight carbon atom alpha-olefins such as ethylene, propylene, vinyl chloride, vinylidene chloride, vinyl fluoride, hexafluoropropylene and chlorotrifluoroethylene. Aromatic monomers such as styrene may also be employed.
By related polymer, it is meant that the polymer repeat unit, or a branch thereof is extended by carbon atoms, preferably from 1 to 4 carbon atoms. For example, a related polymer of acrylic acid is one in which the vinyl group is extended by one carbon to form an allyl group. The polymer salt is an alkali metal salt, and preferably the sodium salt. Still further preferred polymers are those that provide the HF compositions with a thixotropic rheology, so that the compositions are fluid when applied to the substrate surface, but resist flow once at rest.
Preferably, a synthetic water-soluble polymer is used. More preferably, polyacrylic acid or one of its alkali metal salts is used. Most preferably, the water-soluble polymer is sodium polyacrylate.
The polymer may be combined with the HF in any suitable corrosion-resistant vessel. The HF and polymer are blended by shaking, stirring or otherwise mixing the two components in a corrosion-resistant vessel until a uniform, homogenous mixture is obtained. Preferably, a polymer is selected into which the HF dissolves. Because HF has a relatively low vapor pressure, a mixing temperature is selected to minimize HF evaporation. A mixing temperature between about 0 and about 25xc2x0 C. is preferred. Depending upon the viscosity desired for the finished product, the polymer is blended with either anhydrous HF or an aqueous HF solution. One of ordinary skill in the art will understand that the mixing temperature will thus depend upon whether anhydrous HF with a boiling point of 19xc2x0 C. is employed, or aqueous HF with a higher boiling point is used.
The quantity of HF required to perform the desired etch or oxide removal may be calculated and added to the polymer, so that after the etching or oxide removal is complete no residual HF remains. The weight of the substrate to be removed by etching, or the oxide scale to be removed is determined by volumetric measurement, and a stoichiometric quantity of HF is used. For example, with silicon-containing substrates to be etched, SiF4, a gas, is formed. Thus, four moles of HF are employed for every mole of Si to be removed. The number of moles of Si to be removed are determined from the density and Si-content of the volume of substrate to be removed by etching. These calculations are within the ability of the ordinarily skilled artisan. Similar calculations apply to the removal of oxide scale by the formation of fluorine-oxygen compounds.
Etching and oxide scale removal compositions in accordance with the present invention typically contain between about 10 and about 30% by weight HF, and preferably between about 15 and about 25% by weight HF. Anhydrous HF can be employed, or the HF can be mixed with the polymer in the form of an aqueous solution containing HF.
The etching methods of the present invention can be employed with essentially any substrate having an HF-etchable surface. Examples of such substrates include glass, silica, and substrates having a surface coating of glass or silica (such as a video display). The method is particularly useful for etching silica wafers. The method applies an effective amount of the HF composition capable of producing the desired degree of etch to the substrate to be etched for the desired time. Essentially, the stoichiometric amount determined as described above to remove the desired quantity of substrate is applied. The entire surface may be coated or the composition may be applied in a predetermined pattern. The predetermined pattern may be applied with or without a mark or resist covering the areas to remain pattern-free.
Oxide scale removal methods in accordance with the present invention apply an effective amount of the HF composition capable of producing the desired degree of oxide scale removal to an oxidized substrate for the desired time. The method is suitable for use with essentially any substrate that undergoes oxidation. In most situations, the substrate will be an oxidized metal.
The application temperature may be increased or decreased to decrease or increase the etching or oxide scale removal time as desired. As the desired etch or oxide removal is attained, the remaining composition can be removed such as by rinsing or wiping with water. For example, the HF-depleted composition can be rinsed with water, forming an HF waste with much smaller HF content, which optionally can be neutralized by the addition of base. Alternatively, the remaining HF in the HF-depleted composition can be removed by any of the methods disclosed by U.S. Pat. No. 5,766,483, the disclosure of which is incorporated by reference, for example, by heating the composition to about 80xc2x0 to about 125xc2x0 C. using hot gas. This could also be accomplished by heating the substrate. The residual HF that is removed can then be recycled or scrubbed safely by methods that are also disclosed by the above-referenced After either procedure, the residual polymer can then be dried and reconstituted with HF to produce more etching or oxide removal composition.
To attain fine, variable direction lines with the etching method of the present invention, an etching composition having the theological flow properties of an ink can be formulated using polymer ink binders. The etching composition can be drawn in the form of the pattern on a substrate with a pen containing the ink, provided that the pen is assembled from HF-resistant components. Ink-type etching compositions can also be applied to substrates by other essentially conventional printing techniques such as silk screening, direct and offset printing, including gravure printing, with conventional equipment having HF-resistant ink contacting surfaces.
With higher molecular weight polymers (e.g., polymers having weight-average molecular weights of 1,000,000 daltons and higher), compositions may be formulated that are capable of being shaped into a crayon or the xe2x80x9cleadxe2x80x9d of a grease pencil, to provide a writing implement for use with either the substrate etching or oxide scale removal methods of the present invention. Etching methods of the present invention in which the etching composition is applied with a writing implement can be employed in security end-use applications in which parts are etched to indicate an identifying number or code. The etching method can also be employed for freehand drawing of patterns or numbers.
HF compositions may also be prepared with polymers selected to provide the rheological properties of paint, so that the etching and metal oxide scale removal compositions may be applied with a paint brush. Etching and metal oxide scale removal compositions formulated as inks, paints, crayons or grease pencil leads may also contain a dye, ink or other coloring additive to enhance the visibility of the composition on the surface to which it is applied. Such ingredients are conventional and can be readily identified by those of ordinary skill in the art. Etching and oxide scale removal compositions employed in the methods of the present invention may also contain other conventional ink and paint additives.
The invention will be clarified further by a consideration of the following examples that are purely exemplary.